1.Field of Invention
This invention relates generally to optics. It relates particularly to spectacles and eyeglasses. It relates specifically to a variable focal length lens for use in spectacles.
2. Description of Related Art
With age a common problem is that of poor vision for reading. The problem is further complicated if the task is at arms length. For reading, bifocals are needed; for the task at arms length, trifocals are needed. The fields of view are then smaller, presenting annoying coordination problems between the angle of tilt and the center of attention. Going down steps is a problem. Some distance vision is needed, but the bifocal reading segment blurs the picture.
Variable focal length lenses provide an answer to the above problems. Usually they have a variable positive power and are used as additions to prescription lenses. Thus they may not need corrections for astigmatism. However, if variable astigmatism is provided in the lens, then in some cases it is possible to use the variable lens by itself without the presence of a prescription lens.
Variable focus lenses require that changes be produced in the curvatures of the surfaces of the lenses, or that changes be produced in the index of refraction of the optical medium in the interior of the lens. Usually the index of refraction is left constant and the curvatures of the surfaces are changed to provide a spherical power. Several examples of related art for spherical power lenses are given below, and then an example of an early invention of a lens having variable focus and astigmatism will be given.
In U.S. Pat. No. 3,598,479, by Wright a transparent wafer is cemented to the surface of a prescription lens, with a small cavity provided between the wafer and the lens. Provision is made for fluid to be injected into the cavity, deforming the wafer to create a positive power to be added to the power of the prescription lens.
In recent years U.S. Pat. No. 5,138,494, by Kurtin describes a variable focus lens which obtains its variable power by the use of a distensible membrane which is deformed by fluid pressure. The lens comprises the membrane, a supporting ring around the periphery, a glass disc behind the membrane, a cavity filled with fluid, and a rear glass that can provide the front face of a prescription lens; an elastic annular rubber ring serves to contain the liquid in the cavity, and permits the glass disc to move axially. Holes spaced around the periphery of the disc permit the compressed fluid in the cavity to flow into the space between the disc and the membrane, and thus distend the membrane into a spherical shape.
U.S. Pat. No. 5,371,629, provides a non-circular variable focus lens for cosmetic purposes. A non-circular ring supports the membrane around the periphery. A variable elasticity around the ring enables it to cause the membrane deflection under fluid pressure to take a spherical shape and thus produce a variable power. An added cylindrical power is suggested by further modification of the elastic properties of the ring.
The use of a membrane in these two inventions might meet with certain practical problems. The membrane may be easily damaged by small particles. It may be difficult to create uniform tensions in the boundary. The weight of the fluid in the cavity may change the curvature of the surface of the membrane. These are all factors which are difficult to assess. In any case It appears that the two inventions apply primarily to the control of spherical powers.
In U.S. Pat. No. 2,269,905, by Graham, crossed cylinders have been proposed. Variable positive power and astigmatism are achieved. A forward square glass wafer is given a cylindrical curvature by in-plane edge forces and moments applied to the opposite ends of the wafer. A rear square glass wafer is given its curvature by a separately controlled set of forces and moments. The cavity between the two glass wafers is filled with a transparent fluid. The two separately controlled curvatures permits the variable power and astigmatism to be achieved. In the fifty four years since the issuance of the patent, no lenses of this design have appeared on the market. It does seem that reasonably uniform curvature in the wafers would be difficult to achieve by the use of the distributed in-plane edge forces applied in compression to the opposite edges of the wafers. In the present invention to be described below, a completely different forcing system is used to produce reasonably uniform curvatures, and thus a more practical variable lens.